Procedure for the quantification of dust collectability

ABSTRACT

A method and apparatus for determining the collectability of dust particles is disclosed herein. The concept of collectability and collectivity are employed to establish fundamental equations which are solved by the use of data obtained by passing a dust laden gas through a sampling train having a plurality of impingers. The weight of material collected in each separate impinger is determined and using these individual weights a mathematical matrix is set up from which the collectability of the dust particles is obtained. The collectability of a given dust particle is defined as its impinger efficiency when the impinger is operated in strict accordance with design specifications. Collectivity is defined as the collectability distribution of a mixture of dust particles having different collectabilities.

BACKGROUND OF THE INVENTION

One of the problems encountered in the design of pollution controlapparatus is the lack of a standard means to predict accurately the gascleaning efficiency of pollution control devices. For years, pollutioncontrol engineers have used particle size as the main criteria in theircalculations for gas cleaning efficiency. This was particularly true forVenturi Scrubbers. However, particle size alone does not determine gascleaning efficiency and using this measure alone is inadequate forpractical design work.

The method of this invention accomplishes the above by introducing thenew concepts of collectability and collectivity which take into accountall of the variables and mechanisms involved in the wet collectionprocess known as impaction. Heretofore, particle size and particle sizedistribution were the main criteria in design calculations. Particlesize alone, however, does not determine gas cleaning efficiency andVenturi pressure drop. Other particle characteristics such as shape,density and electrostatic charge must be taken into consideration. Aterm called "aerodynamic particle diameter" takes into account shape anddensity but not electrostatic charge.

The present invention comprises a method and apparatus for accuratelyquantifying the collectivity of a mixture of dust particles with variouscollectabilities. Knowing the collectivity of the dust mixture in a dustladen air/gas stream as described in this invention, the following canbe accomplished:

1. The collectability of the dust particles will be expressed aspositive fractions between zero and one.

2. The collectivity of the dust mixture will be reported in a table or agraph showing weight fraction versus collectability.

3. By determining the collectivity of a dust from a dust generatingprocess, the Venturi pressure drop required for any gas cleaningefficiency or outlet dust concentration can be determined without thenecessity of making pilot plant studies.

4. By testing the inlet and the outlet gas streams of a pollutioncontrol system, its comparative performance will be known. Knowing thecollectivity of the inlet and outlet gas streams, not only the system'sactual efficiency on particles of all collectabilities can be calculatedbut these actual efficiencies can be compared to those efficiencies ofall other systems tested in this manner even if the systems are ondifferent applications. Thus, we have a means for comparing gas cleaningsystems on boilers, blast furnaces, kilns, incinerators and all otherapplications.

5. Obviously, liquid entrainment can be determined by the gain of waterin the first impinger if other gains/losses are taken into account. Butmore important, the amount of particulate matter in this liquidentrainment can be determined using the concept of collectability.

Specifically, there is no relevant prior art of record since applicantis disclosing a new concept herein. Consequently, the cited prior art ismerely of marginal interest although the invention may lie in the samegeneral field. In prior art, cascade impactors have been used todetermine particle size but the present invention relates tocollectability as defined herein.

In the prior art, Davis et al U.S. Pat. No. 4,509,727 discloses anoff-gas monitor for determining gas scrubber efficiency in steelprocesses. The method comprises comparing the particulate matter in agas stream before and after chemical treatment to determine theefficiency of the treatment.

Tarcy U.S. Pat. No. 4,479,379 discloses a method and apparatus forcontinuous analysis of a gas and particulate stream. A sampling nozzleis placed in a gas and particulate stream and a solvent is injectedcreating an aerosol which enhances dissolution of gas in the solvent. Acontinuous analysis means examines the concentration of dissolved gasand particulate.

Also of interest are prior art Vaneldik U.S. Pat. No. 3,705,478 andMaatsck U.S. Pat. No. 3,457,787. The prior art patents show differentmeans for measuring the concentration of particles in a gas and indeedin the environment of pollution control devices such as scrubbers andprecipitators. None of the patents discloses the apparatus and method ofthe present invention.

SUMMARY OF THE INVENTION

The present invention comprises a method and apparatus for quantifyingthe collectability of dust particles in a gas stream. The methodutilizes the concept of "collectability" and "collectivity" to accountfor all of the variables and mechanisms involved in the wet collectionprocess known as impaction. The apparatus includes a sampling trainwhich comprises an impinger train operating under specific conditionsand comprising a plurality of impingers in series which are accuratelycalibrated with known dusts. The weight of material collected in eachseparate impinger is determined and using these individual weights amathematical matrix is set up from which the collectability of the dustparticles is obtained. "Collectability" is the efficiency of collectionof a given particle in a specially designed and operated impinger whilethe term collectivity is an expression of the collectabilities of allparticles in a dust mixture.

In operation, the particulate laden gas stream is passed through a trainof impingers. Knowing the weight of dust collected in each impinger andalso the weight passing through the impinger train, the weight fractionof dust in a number of classes (ranges) of collectabilities can bedetermined.

The number of classes is the number of impingers in the train. Inpractice 10 impingers and 10 classes have been chosen but any number canbe used. A particle with a specific collectability has a specific curverelating gas cleaning efficiency with Venturi pressure drop. Likewise,for an electrostatic precipitator this particle will have a specificcurve relating gas cleaning efficiency with voltage, retention time andmigration velocity.

Accordingly, it is an object of this invention to provide a new andunique method and apparatus for quantifying the collectability of dustparticles in a gas.

A further object of this invention is to provide a new and improvedmethod for measuring the efficiency of air pollution control apparatus.

A still further object of this invention is to provide a new andimproved apparatus for measuring the efficiency of air pollution controlapparatus.

A more specific object of this invention is to provide a new method andapparatus for determining collectability by passing a dust laden gasthrough a sampling train having a plurality of impingers so that theweight of dust particles in each impinger can be obtained, and using amathematical matrix, the efficiency of the impinger on individualparticles can be determined. The invention also introduces the conceptof collectivity which is defined as the collectability distribution of amixture of dust particles having different collectabilities.

DESCRIPTION OF THE DRAWINGS

The above and other objects of the invention may be more clearly seenwhen viewed in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of the apparatus for practicing the methodof the invention;

FIG. 2 is a cross-sectional view of a typical impinger in the impingertrain; and

FIG. 3 is a crossection view of the probe for sampling the gas stream.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, the invention comprises a method andapparatus for determining the collectability of dust particles in a gas.Collectability is a dimensionless number between zero and one and it isequal to the efficiency of collection of a given particle in a specialimpinger. Another concept introduced by this invention is "collectivity"which is an expression of all the collectabilities of a given dustmixture. Collectivity can be expressed in tabular form in which a smallrange of collectability is given versus weight percent in that range orit can be expressed (as in particle size distribution) as "weightpercent less than" a certain collectability or it may be expressed inequation form.

The sampling and testing procedure of this invention was developed toprovide the following:

1. Provide information about the characteristics of a dust in additionto the dust concentration from a single dust sample.

2. Provide a direct means of evaluating the relative performance of anygas cleaning system.

3. Provide a means of determining the pressure drop required for any gascleaning efficiency of a Venturi Scrubber.

4. Provide a sampling procedure in which the sampling probe:

(a). will not retain a significant portion of the dust and

(b). will not require movement to several positions in the duct foriso-kinetic sampling.

5. Determine entrainment of scrubbing liquid and particulate mattertherein.

As shown in FIG. 1, the sampling train 10 of the present inventioncomprises a duct 11 through which a dust laden gas stream 15 passes inthe indicated direction. The sampling probe 12 includes a minimum of sixsampling ports 13. The location of the probe 12 is critical since it isequivalent to a single traverse of a conventional one-port probe.

The probe 12 is coupled to a metering Venturi 14 with an outletextraction tube 16. A control valve 17 and a blower 18 are also coupledto the Venturi 14 in an arrangement which measures and controls gas flowunder iso-kinetic conditions. The flow from the extraction tube 16, alsounder iso-kinetic conditions from the Venturu 14, passes to an impingertrain 19 which in a preferred embodiment comprises ten impingers inseries. The flow from the blower 18 is reintroduced to the duct 11through tube 18a.

The individual impingers 20 (FIG. 2) collect decreasing amounts of dustas the gas stream passes from the first to the last impinger. The streamthen passes through a final filter 22 to a cooler-condenser-desicant 23,and a dry test meter 24. A vacuum pump 26 maintains the flow of gas anda flow meter 27 measures that flow.

In the sampling procedure, the metering Venturi 14 and blower 18 arestarted and adjusted to obtain iso-kinetic sampling conditions in themain duct 11 by maintaining a specified pressure as measured bymanometer 10. The Venturi 14 and blower 18 are run for a minimum of onehour before starting the sampling train 19. The gas flow through themetering Venturi is an order of magnitude greater than that through theimpinger train 19. After an initial build-up of dust in the probe 12,further dust accumulation ceases and probe dust catch can usually beneglected. Nevertheless, the probe 12 is always washed and the dustaccumulation determined after every test. If the accumulation happens tobe significant, it can be added to the final result.

After the proper sampling conditions are reached, the impinger train 19is started and the gas flow adjusted to stated conditions. Sampling canbe continued as long as necessary to obtain measurable amounts ofcontaminants (dust) in the last impinger 20. Since most of the dust iscaught in the impingers 20, the final filter 22 is seldom overloaded andneed not be changed during the test. While a preferred embodiment wouldcomprise 10 impingers, any number can be used with the results andstructure of the train differing.

After the sampling period, the final weight of material collected ineach impinger 20 is determined. The total weight of material passingthrough the impingers 20 is determined by adding the weight of materialcollected on the filter to the sum of all impinger weights. Since theindividual weights are known, the collection efficiency of each impinger20 can be calculated.

However, overall impinger efficiency is not the same as collectabilitybut having all of these quantities and variables, it is possible toobtain curves (graphs) showing "percent by weight less than" a certaincollectability or "percent by weight" in a given range ofcollectabilities. Any of these curves thus constitute the collectivityof the dust particles.

Testing shows that the material in the clean gas from most pollutioncontrol systems contains a large percentage by weight of material with acollectability of one (1.0000). The term "debris" is used to describethis material. It is material that requires no Venturi pressure drop forcollection in a Venturi scrubber or no applied voltage in anelectrostatic precipitator. In a wet system, it is entrainment or mistwhich can carry soluble material as well as particulates, bothsub-micron and plus-micron. In the present method this material is amajor portion of the catch of the first impinger and can be reportedseparately. The balance of the particulates are then collected in thefollowing impingers and final filter.

A typical impinger 20 is shown in FIG. 2. The impinger 20 comprises asubstantially cylindrical container 21 with the extraction tube 16extending downwardly through an aperture 20a in the cap 20b and beingspaced a predetermined distance above the water 33 in the bottom of thecontainer 21. The outlet diameter 34 of the tube 16 and the distance 36of the tube above the water are two important constants.

The lower portion of the container 21 is connected via tube 25 to thereservoir 28 which includes an upper outlet tube 29 which is connectedto the upper portion of the container 21. The water 24 in the reservoir28 is used to raise or lower the water level in the container 21 inorder to maintain the distance of the tube 16 above the water 24. Thisis done by raising or lowering vessel 28 in relation to container 21 byuse of flexible tubing for 25 and 29. The valve 35 is used to supplywater to the reservoir 28. Clamps or other controls may be used tocontrol the water level. Typically, the hot or saturated gas enters theimpinger 20 through tube 16 and exits through upper opening 31 to thetube 32 and then to the next of the serially connected impingers 20. Theweight of the dust collected in the impingers is obtained by filteringthe liquid and weighing the material on a filter paper. This is done foreach of the impingers and the results fed into a computer programmedwith equations to provide the collectivity.

An individual dust particle has a specific collectability which is itsefficiency of collection in a standardized dust sampling device, theimpinger 20 as described above. On the other hand, collectivity is theknowing or determination of all the combined collectabilities of all ofthe individual dust particles of a dust mixture. Thus collectability isanalogous to individual particle size while collectivity is analogous toparticle size distribution.

The sampling device disclosed herein is an impinger 20 with a givenfixed jet diameter operated at a fixed pressure drop and a fixed jetimpaction parameter which may be represented by the equation: ##EQU1##where: u=jet velocity

ρ=particle density

D_(p) =particle diameter

μ=gas viscosity

D_(j) =jet diameter

C=Cunningham corrector factor

For a sampling train 19 containing, for example a plurality of fiveimpingers, one obtains five equations and five unknowns as follows:

    W.sub.1 =E.sub.a x.sub.a +E.sub.b x.sub.b +E.sub.c x.sub.c +E.sub.d x.sub.d +E.sub.e x.sub.e

    W.sub.2 =I.sub.a E.sub.a x.sub.a +I.sub.b E.sub.b x.sub.b +I.sub.c E.sub.c x.sub.c +I.sub.d E.sub.d x.sub.d +I.sub.e E.sub.e x.sub.e

    W.sub.3 =I.sub.a.sup.2 E.sub.a x.sub.a +I .sub.b.sup.2 E.sub.b x.sub.b +I.sub.c.sup.2 E.sub.c x.sub.c +I.sub.d.sup.2 E.sub.d x.sub.d +I.sub.e.sup.2 E.sub.e x.sub.e

    W.sub.4 =I.sub.a.sup.3 E.sub.a x.sub.a +I .sub.b.sup.3 E.sub.b x.sub.b +I.sub.c.sup.3 E.sub.c x.sub.c +I.sub.d.sup.3 E.sub.d x.sub.d +I.sub.e.sup.3 E.sub.e x.sub.e

    W.sub.5 =I.sub.a.sup.4 E.sub.a x.sub.a +I .sub.b.sup.4 E.sub.b x.sub.b +I.sub.c.sup.4 E.sub.c x.sub.c +I.sub.d.sup.4 E.sub.d x.sub.d +I.sub.e.sup.4 E.sub.e x.sub.e

E_(a).sbsb.1, E_(b), etc. =average collectability of a class ofparticles, whose individual collectabilities lie in a narrow range, e.g.0.99 to 1.00 or 0 to 0.01. These ranges can be set arbitrarily but mustcover the span 0 to 1.

W₁, W₂, etc.=weight fraction collected in each impinger

x_(a), x_(b), etc.=weight fraction of class a,b,etc. particles(collectability distribution or collectivity)

x_(f) =Weight fraction caught on filter

I_(a) =1-E_(a), I_(b) =1-E_(b), I_(c) =1-E_(c), etc.

Then:

    Total wt collected=W.sub.1 +W.sub.2 +W.sub.3 +W.sub.4 +W.sub.5 +x.sub.f =1

    Total wt collected=x.sub.a +x.sub.b +x.sub.c +x.sub.d +x.sub.e +x.sub.f =1

This is one way of expressing collectivity.

    ______________________________________                                        Known quantities:  W.sub.1,W.sub.2, W.sub.3,W.sub.4,W.sub.5,x.sub.f                              E.sub.a,E.sub.b,E.sub.c,E.sub.d,E.sub.e                    Unknown quantities:                                                                              x.sub.a,x.sub.b,x.sub.c,x.sub.d,x.sub.e                    ______________________________________                                    

The unknown quantities are obtained by solving the above five equations.These equations form a matrix which can be solved through the use ofdeterminants. If the range of E_(a) is set so that it approaches one,e.g. 0.96 to 1.0, the fraction x_(a) of the dust will have highcollectability and will require a low pressure drop for high efficiencyof collection in a Venturi Scrubber. If the range of E_(e) is set sothat it approaches zero, e.g. 0-0.04, the fraction x_(e) will have amuch lower collectability and will require a high pressure drop forcollection with a reasonable efficiency.

It is understood that the above described arrangements are merelyillustrating examples of the invention. Numerous other arrangements maybe readily devised by those skilled in the art which will embody theprinciples of the invention and fall within the spirit and scopethereof.

What is claimed is:
 1. The method of quantifying the collectability ofdust particles in a gas stream which comprises the steps of:passing adust laden gas through a sampling train having a plurality of impingers,collecting the dust particles in each impinger as the gas passes throughthe impinger train, weighing the dust particles collected in eachimpinger, and setting up mathematical equations using the weights ofdust collected in each individual impinger from which the collectabilitydistribution of the dust particles is obtained.
 2. The method of claim 1further including the steps of:setting up a mathematical matrix fromwhich the collectivity of the dust particles is obtained.
 3. The methodof claim 1 further including the steps of:obtaining a sample of a gasstream by means of a probe inserted into the gas stream, controlling theflow of the gas stream sample under iso-kinetic conditions to theimpinger train, and filtering the sampled gas from the impinger train tocapture any dust which has escaped the impingers.
 4. The method of claim1 wherein:the gas is passed through an impinger train wherein theimpingers are connected in series.
 5. The method of accuratelyquantifying the collectivity of a mixture of dust particles with variouscollectabilities comprising the steps of:providing a plurality of dustcollecting devices in a series-connected sampling train, directing arepresentative sample of a dust laden gas stream from a main duct to thedust collecting devices, weighing the dust collected in each device,setting up a mathematical matrix from which the collectability of thedust particles is obtained, and determining the collectivity of the dustmixture from the weights of dust collected in each of the devices. 6.The method of claim 5 wherein:the dust collecting devices comprise aplurality of impingers.